Corrosion behaviour of garnet particulatereinforced LM13 Al alloy MMCs

K.H.W. Seah a,*, M. Krishna b, V.T. Vijayalakshmi c, J. Uchil d

a Department of Mechanical and Production Engineering, National University of Singapore, 10 Kent

Ridge Crescent, Singapore 119260, Singaporeb Department of Mechanical Engineering, R.V. College of Engineering, Bangalore-560059, India

c Department of Chemistry, N.M.K.R.V. College for Women, Jayanagar, Bangalore-560011, Indiad Department of Material Science, Mangalore University, Mangalagangothri, Karnataka, India

Received 22 May 2000; accepted 23 May 2001

Abstract

This paper describes a study of the corrosion characteristics of LM13 Al alloy-based

composites reinforced with various amounts of garnet particulates. The weight loss method

was used and the corrodent was 1 M HCl solution at room temperature. The durations of the

tests ranged from 24 to 96 h. Corrosion tests were performed on the unreinforced matrix alloy

as well as on the various composites in both heat-treated and as-cast conditions. In each test,

the corrosion rates of the unreinforced matrix alloy and the composites were found to decrease

with duration of exposure to the corrodent. Solution heat treatment at 525C followed byarticial aging at 175C was found to improve the corrosion resistance of every specimentested. Corrosion resistance was also found to improve with increase in garnet content. An

attempt is made in the paper to explain these phenomena. 2002 Elsevier Science Ltd. Allrights reserved.

Keywords: LM13; Aluminium alloy; Garnet; Corrosion; Metal matrix composite

1. Introduction

Metal matrix composites (MMCs) oer designers many benets as they areparticularly suited for applications requiring good strength at high temperature,good structural rigidity, dimensional stability, and light weight [15]. The trend is

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Corrosion Science 44 (2002) 917925

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E-mail address: mpeskh@nus.edu.sg (K.H.W. Seah).

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towards safe usage of the MMC parts in the automobile engine, which work par-ticularly at high temperature and pressure environments [6,7]. Particulate reinforcedMMCs have been the most popular MMCs over the last two decades. Of these,ceramic reinforced Al-based MMCs are the most common. Although the incorpo-ration of a second phase into a matrix material can enhance the physical and me-chanical properties of the latter, it could also signicantly change the corrosionbehaviour.

Particulate reinforced Al-based MMCs nd potential applications in severalthermal environments, especially in automobile engine parts such as drive shafts,cylinders, pistons, and brake rotors. Al-based MMCs which are used in automobileengine parts normally encounter acidic environments containing chloride, sulphiateand nitrate radicals, in addition to exhaust gases like CO2, CO and NOx. MMCsused at high temperatures should have good mechanical properties and resistance tochemical degradation in air and acidic environments [8]. For high-temperature ap-plications, it is essential to have a thorough understanding of the corrosion behav-iour of the aluminium composites. Published data [911] indicate that the addition ofSiC particles do not appear to improve corrosion resistance on some aluminiumalloys because pits were found to be more numerous on the composites than on theunreinforced alloys although they were comparatively smaller and shallower thanthose on the unreinforced alloy. Gonzalez et al. [12] reported that the presence of SiCparticles does not give rise to signicant galvanic corrosion and no active phases areformed at the matrix/particle interface. Nevertheless, the present study is aimed atcharacterising the corrosion behaviour of LM13 Al alloy and the eect of reinforcingit with garnet particles in both the as-cast and the heat-treated conditions.

2. Experimental procedure

2.1. Constituent materials of composites

The matrix alloy used for the composites is LM13 Al alloy, a material suitable formass production of lightweight castings which can either be sand-cast or die-cast.The chemical composition of the LM13 alloy is given in Table 1.

Garnet, the reinforcement material, is abundantly available in the earth and hasa Mohrs hardness of 6.57.0 which is nearly equal to that of SiC. It is composedof alumino-silicates of calcium, having the chemical formula Ca2Al2(SiO4)3 and ischemically inert at elevated temperatures. It does not have a sharp melting pointalthough it softens at temperatures of 11401280C.

Table 1

Chemical composition of matrix LM13 alloy by weight percentage

Mg Si Fe Cu Ti Pb Zn Mn Sn Ni Al

0.81.5 1012 1.0 0.71.5 0.2 0.1 0.5 0.5 0.1 1.5 balance

918 K.H.W. Seah et al. / Corrosion Science 44 (2002) 917925

sahibHighlight

2.2. Preparation of the composites

The liquid metallurgy technique was adopted to prepare the LM13-based com-posites. In this technique, a measured quantity of preheated garnet particulates wasslowly added into the vortex created in the molten matrix alloy by stirring it with analumina-coated steel impeller in an inert gas atmosphere. The coating of alumina isnecessary in order to prevent the migration of ferrous ions from the stirrer to thematrix alloy melt. The melt containing the reinforcement was then poured into thelower half of the die (permanent mould) and the top half was brought down to applypressure on the melt as it solidied. The unreinforced matrix material was cast in thesame manner, except that no garnet particulates were added. LM13-based com-posites containing 2%, 4% and 6% by weight respectively of garnet particulates werefabricated and tested, and their properties were compared with those of the unre-inforced matrix alloy.

2.3. Specimen preparation

Cylindrical specimens of diameter 20 mm and height 20 mm were machined fromcastings of the composite and the base alloy. Before corrosion testing, the specimensurfaces were ground with 1000 grit silicon carbide paper and then polished using 3lm diamond paste to obtain a good surface nish. The specimens were then washedin distilled water, followed by acetone, and then allowed to dry thoroughly.

2.4. Heat treatment and aging

To study the eect of articial aging on the composites, some of the as-cast testspecimens were subjected to solution heat treatment at 525C for 48 h, quenched inwater at 80C and then aged at 175C for 4, 8 and 12 h, respectively.

2.5. Corrosion testing

The corrosion tests were static immersion tests conducted at room temperatureusing the conventional weight loss method to an accuracy of 0.1 mg. Each specimenwas rst weighed before being immersed in 200 ml of 1 M HCl solution and latertaken out after 24, 48, 72, and 96 h respectively. HCl solution was selected as thecorrodent for accelerated results since the corrosion studies reported in this paper arecomparative ones. After each corrosion test, the specimen was immersed in Clarkssolution for 10 min and gently cleaned with a soft brush to remove adhered scales.Clarks solution is a standard mixture containing potassium chloride, potassiumphthalate, potassium phosphate, boric acid and sodium hydroxide [13]. After dryingthoroughly, the specimens were weighed again. The weight loss was measured andconverted into corrosion rate expressed in mils penetration per year (mpy). Thecorroded surfaces were studied using scanning electron microscopy (SEM).

K.H.W. Seah et al. / Corrosion Science 44 (2002) 917925 919

3. Results and discussion

3.1. Microscopy

Fig. 1(a) shows that the unreinforced matrix alloy has a very thin discontinuouslayer on the surface with small dendrites growing. The photographic image is de-pendent principally on the mean atomic number of the dierent phases present in thematrix alloy. Thus, the aluminium-rich regions are bright whereas the other elements(Mg, Si, Fe, Cu, etc.) turn out dark in the optical microscope.

A typical photomicrograph of the transverse section of an as-cast MMC specimencontaining 6% by weight of garnet is shown in Fig. 1(b). The distribution of par-ticulates appears to be reasonably homogenous, although there was a tendency forthe particulates to be aligned more in the longitudinal direction.

Fig. 1. (a) Photomicrograph showing dendritic structure of unreinforced LM13 alloy. (b) Microstructure

of LM13-based MMC reinforced with 6% garnet.

920 K.H.W. Seah et al. / Corrosion Science 44 (2002) 917925

3.2. Eect of duration of exposure to corrodent

Table 2 shows the results of all the corrosion tests conducted on the unreinforcedmatrix alloy and on the MMCs in the as-cast and heat-treated conditions in 1 M HClsolution. These results are represented graphically in Figs. 2 and 3. Each resulttabulated is an average obtained from six identical tests. In each case, the error bars(not shown) are 35% from the mean value.

Fig. 2 shows plots of the corrosion rate (in mpy) of the as-cast unreinforcedmatrix alloy and composites in HCl solution against exposure time (in hours), as wellas for the same materials aged for 4, 8 and 12 h respectively. It can be seen that inevery case, there is a decrease in corrosion rate with increase in duration of exposureto the corrodent, implying that the corrosion resistance of the materials tested in-creases as the exposure time is increased. Visible inspection showed that there wereno hydrogen bubbles clinging onto the surface of the test specimens. The phenom-enon of monotonically decreasing corrosion rate with respect to time indicates somepassivation of the matrix alloy. De Salazar et al. [14], in their study of alumina-reinforced Al-based MMCs, observed that a protective black lm is formed on thesurface consisting of hydrogen hydroxy chloride, which apparently retards the cor-rosion. Castle et al. [15], who studied SiC particulate reinforced Al-based MMCs,believe that the black lm formed on the surface consists of an aluminium hydroxidecompound which protects the bulk material from further corrosion in the acidmedium. The exact chemical nature of such a protective lm is still not fully

Table 2

Corrosion rate of as-cast and heat-treated LM13 alloy and MMCs for dierent durations of exposure to

HCl solution

Aging time (h) Garnet con-

tent (wt.%)

Corrosion rate (mpy)

After 24 h

exposure

After 48 h

exposure

After 72 h

exposure

After 96 h

exposure

0 0 10.93 5.64 4.12 3.21

2 10.80 5.52 4.02 3.11

4 9.15 5.07 3.52 3.06

6 8.66 4.68 3.55 3.00

4 0 7.97 4.47 3.25 2.97

2 7.47 4.24 3.16 2.83

4 7.18 4.20 2.82 2.41

6 6.60 4.47 2.79 2.31

8 0 7.72 3.65 2.88 2.30

2 7.38 3.63 2.83 2.12

4 6.16 3.60 2.72 2.11

6 6.40 3.58 2.91 2.10

12 0 7.68 3.98 2.83 2.22

2 7.09 3.10 2.56 2.02

4 5.99 3.37 2.67 2.01

6 5.79 3.35 2.69 1.95

K.H.W. Seah et al. / Corrosion Science 44 (2002) 917925 921

understood. However, it can be seen in each graph that as duration of exposure isincreased, the benecial eect on corrosion resistance levels o, probably due to theprotective layer reaching a steady state with time.

3.3. Eect of heat treatment

Fig. 3 is a plot of corrosion rate after 24 h exposure time against the duration ofaging for the unreinforced matrix alloy and the MMCs. It can be seen that aging for4, 8, and 12 h (at 175C) signicantly improves the corrosion resistance of everyspecimen tested. Solution heat treatment and aging is used to improve materialstrength for precipitation-hardening alloys. It can be seen that aging for 4 h causes

Fig. 2. Graphs of corrosion rates of LM13 alloy and MMCs vs. exposure time in 1 M HCl solution for

various durations of aging.

922 K.H.W. Seah et al. / Corrosion Science 44 (2002) 917925

the corrosion rate to drop by about one-third. One possibility is that the articialaging has enhanced the protective layer of aluminium oxide described above, con-ferring greater corrosion resistance to the corrodent which is seen in all the speci-mens, unreinforced as well as reinforced, no matter what the garnet content is.As aging duration is increased beyond 4 h, the benecial eect on corrosion resis-tance apparently levels o, probably due to the protective layer of aluminium oxidereaching a steady state with time during aging, which was done after the specimenhad been machined.

3.4. Eect of garnet content

From Table 2 and Fig. 2, it is apparent that for materials in both the as-cast andaged conditions, there is a trend of decreasing corrosion rate with increase in garnetcontent, especially for shorter exposure times. For long exposure times, however, thiseect is less pronounced.

The corrosion rate of the unreinforced matrix alloy is higher than those of thecomposites because in the former, there is no reinforcement phase and the matrixalloy does not have much corrosion resistance to the acid medium. Garnet, being aceramic, remains inert and is itself unaected by the acidic medium during the tests.The inert garnet particulates are also not expected to aect electrochemically thecorrosion mechanism of the composite. Nevertheless, the results show an improve-ment in corrosion resistance as the garnet content is increased in the composite,indicating that the garnet particulates do inuence the corrosion characteristics ofthe composites albeit not electrochemically. Sharma et al. [16] obtained similar

Fig. 3. Eect of articial aging on corrosion rate of LM13 alloy and MMCs after 24 h of exposure in 1 M

HCl solution.

K.H.W. Seah et al. / Corrosion Science 44 (2002) 917925 923

results in short glass ber reinforced ZA-27 aluminium alloy composites, observingthat the corrosion resistance increases with increase in reinforcement content.

Wu Jianxin et al. [17] in their work on corrosion of SiC particulate reinforced Al-based MMCs state that this corrosion behaviour is not aected to a signicant extentby the presence of the SiC, although these particulates denitely play a subsidiaryrole as physical barriers to MMC corrosion. According to them, particulates act asinert physical barriers to the initiation and development of pitting corrosion, mod-ifying the microstructure of the matrix material and hence improving the corrosionresistance of the MMC.

Another reason for the decrease in the corrosion rate could be the formation of amagnesium inter-metallic layer adjacent to the particle during fabrication of thespecimen as discussed by Trzaskoma [9]. McIntyre et al. [18], in their research onMMCs, showed that the magnesium inter-metallic compounds are more active thanthe alloy matrix causing more pitting in the particle/matrix interfaces because of thehigher magnesium content in such regions. These electrochemically active crevicesact as sacricial anodes and protect the rest of the matrix, restricting pit formationand propagation to only these crevices [19]. Further evidence of the formation of amagnesium layer can be found in another paper by Sharma [20].

4. Conclusions

In the present study of garnet particulate reinforced LM13 Al alloy MMCs,corrosion resistances of the unreinforced matrix alloy and the composites were foundto improve with duration of exposure to the corrodent. Solution heat treatment at525C followed by articial aging at 175C was found to improve the corrosionresistance of every specimen tested. The improvement in corrosion resistance due tothese two factors is attributed to a protective layer formed on the surface of thematerial. Corrosion resistance was also found to improve with increase in garnetcontent, probably due to the garnet particles acting as physical barriers to the cor-rosion process.

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